(1) Technical Field
This disclosure relates generally to the field of audio amplifier circuits and relates more specifically to a fully differential class AB amplifier topology.
(2) Background
Power audio amplifiers are widely used to drive speakers in audio systems. Different classes of audio amplifiers are utilized to provide output signals. For example, class A amplifiers reproduce an entire input signal because an active element is constantly in active mode, hence having high power consumption.
To improve the full power efficiency of the previous Class A amplifier by reducing the wasted power in the form of heat, it is possible to design the power amplifier circuit with two transistors in its output stage producing what is commonly termed as a “push-pull” type amplifier configuration. Push-pull amplifiers use two “complementary” or matching transistors, one being an NPN-type and the other being a PNP-type with both power transistors receiving the same input signal together that is equal in magnitude, but in opposite phase to each other. This results in one transistor only amplifying one half or 180 degrees of the input waveform cycle while the other transistor amplifies the other half or remaining 180 degrees of the input waveform cycle with the resulting “two-halves” being put back together again at the output terminal. Then the conduction angle for this type of amplifier circuit is only 180 degrees or 50% of the input signal. These types of audio amplifier circuits are more generally known as the Class B Amplifier.
The Class AB Amplifier circuit is a compromise between the Class A and the Class B configurations. Both transistors slightly conduct even when no input signal is present.
In the class-D amplifier the input signal is converted to a sequence of higher voltage output pulses. The averaged-over-time power values of these pulses are directly proportional to the instantaneous amplitude of the input signal.
Class-G amplifiers use “rail switching” to decrease power consumption and increase efficiency. These amplifiers provide several power rails at different voltages and switch between them as the signal output approaches each level. Thus, the amplifier increases efficiency by reducing the wasted power at the output transistors.
The most common output driver modes are class AB and class D type; class AB uses the power stage devices in linear mode while class D uses the power devices in switched mode
Current practice uses rail-to-rail intermediate stage to drive high/low side power devices. This approach has multiple disadvantages:                In general, battery or output of a charge pump is used to supply the power stage, those same noisy supplies need to propagate to the intermediate stage, feeding noise and compromising PSR. One example of this mechanism is feed-forward through Miller compensation capacitors.        When the supply is varying widely, like in class-G operation, its generally much more difficult to keep the intermediate stage biased in linear mode, requiring complex and larger design to arbitrate between adaptive (sometimes concurrent) controls inside the intermediate stage.        When cascaded low voltage PMOS devices are used, it's generally much more complicated in those topologies to protect their gates from seeing the full supply, especially at start-up.        
It is a challenge for engineers to design an audio amplifier the disadvantages cited above.